A century ago, mathematician J. Radon demonstrated that a two-dimensional slice of a three-dimensional object may be reproduced from the set of all of its projections. Computed tomography (CT) X-ray systems generate a set of X-ray beam projections through an object to be examined. The resultant detected X-ray data are computer processed to reconstruct a tomographic image-slice of the object.
Conventional CT systems subject the object under examination to one or more pencil-like X-ray beams from all possible directions in a plane. The X-ray data may be generated in fan beam format (as is the case for the present invention), or in parallel beam format. In a fan beam system, the X-rays radiate from a source and are collected in a fan. By contrast, in a parallel beam system the X-rays are all parallel within a view. In either system, a view is one projection of the object onto the detectors, and a scan is a collection of all of the views.
In a fan beam scanning electron beam system such as described in U. S. Pat. No. 4,521,900 to Rand, or U.S. Pat. No. 4,352,021 to Boyd, an electron beam is produced by an electron gun and is accelerated downstream along the z-axis of an evacuated chamber. Further downstream a beam optical system deflects the electron beam about 30° into a scanning path, with azimuthal range typically about 210°. The deflected beam is then focused upon a suitable target, typically a large arc of tungsten material, which produces a fan beam of X-rays.
The emitted X-rays penetrate an object (e.g., a patient) that is disposed along the z-axis and lying within a so-called reconstruction circle. X-ray beams passing through the object are attenuated by various amounts, depending upon the nature of the object traversed (e.g., bone, tissue, metal). One or more X-ray detectors, disposed on the far side of the object, receive these beams and provide signals proportional to the strength of the incoming X-rays.
Typically the output data from the detectors are processed using a filtered back-projection algorithm. Detector data representing the object scanned from many directions are arranged to produce image profiles for each scan direction. Since the X-rayed object is not homogeneous, these profiles will vary in intensity with the amount of radiation detected by the various detectors on the various scans. The convoluted data from the various projections are then superimposed, or back-projected, to produce a computed tomographic image of the original object. The thus processed data are used to produce a reconstructed image of a slice of the object, which image may be displayed on a video monitor.
Systems similar to what is described in the above patents to Rand or Boyd are manufactured by Imatron, Inc., located in South San Francisco, Calif. These systems are termed “short scan” because the views used for reconstructing an object image cover 180° plus the fan beam angle (about 30°), e.g., about 210° total, rather than a full 360°. In a scanning electron beam CT system, the 210° angle implies that the target and detector must overlap, which is to say occupy the same space azimuthally.
In prior art systems this problem has been solved by using a parallel planar target and detector that are separated axially. In such systems, the X-ray detectors also span 180° plus the fan angle, and define a first plane that is orthogonal to the z-axis. The source of the X-rays scans or travels within a second plane, also orthogonal to the z-axis, but not coincident with the first plane. As the X-ray fan rotates around the target, the central ray from the target to the detector describes a shallow cone rather than an ideal plane. Thus, although ideally reconstruction creates an image in a plane perpendicular to the z-axis using views acquired within that plane, in practice the presence of a cone angle in prior art systems results in each acquired view being inclined rather than perpendicular to the z-axis. This “cone beam” effect causes data sets acquired from axially non-invariant objects to be self-inconsistent, which results in image artifacts that degrade image quality and can produce false diagnoses in medical applications.
Unless the cone beam geometry is accounted for by using data from more than one axial position, cone beam error results. The result is a reconstructed image that includes unwanted cone beam artifacts that appear as streaks in the reconstructed, displayed image. In general, cone beam artifacts can be reduced in part only at the expense of scanning contiguous or overlapping slices, and interpolating the data from adjacent slices. U.S. Pat. No. 5,406,479 (1995) to Harman, assigned to Imatron, Inc., assignee herein, describes a method of reconstructing data acquired from a fan beam system such that cone beam error is substantially reduced. However efficient as the Harman technique is, it still requires data processing steps that would not be required if cone beam error could simply be eliminated as an error source. Applicants refer to and incorporate by reference U.S. Pat. No. 5,406,479 to Harman, U.S. Pat. No. 4,521,900 to Rand, and U.S. Pat. No. 4,352,021 to Boyd.
Another problem associated with prior art scanning electron beam CT systems occurs in so-called helical or spiral scanning. In this mode of operation, an object is continuously scanned while being moved at a constant velocity in the axial direction. During scanning, azimuthal and axial motions of the X-ray fan become mixed such that further interpolations must be performed on the resultant data before planar images can be reconstructed. The requirement to perform data reconstruction slows down the reconstruction process, reduces system throughput, and adds to the processing requirements of the overall system.
Thus in a scanning electron beam CT system, there is a need for a method and system to substantially eliminate cone beam error, and thus a need to compensate or correct for such error. Preferably the resultant system should exhibit higher quality images with reduced false diagnoses in medical applications. Finally, the reduction in data processing realizable by such systems should make possible scanning of continuously moving objects with image reconstruction substantially in real-time.
The present invention provides such a system.